ts34118 Taiwan Semiconductor Company, Ltd. (TSC), ts34118 Datasheet - Page 17

• Manufacturer Part Number: ts34118
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: Taiwan Semiconductor Company, Ltd. (TSC)

TS34118
Design Equations
The “M” terms are the slopes of the lines (0.52,0.024, and 0.0019) which are the coefficients of the three equations.
The M
switch to transmit in the presence of a given receive signal level. The M
threshold, The M
necessary to overcome noise on the other.
Some comments on the above graph:
Equation 13 is independent of the volume control setting. Conversely, the acoustic coupling of a designed system helps
determine the minimum slope of that line. Using the component values in Equation 13 yields a G
Experience has shown, however, that an acoustic coupling loss of >40dB is desirable.
Using the component values in Equation 14 yields a maximum side tone of 0dB. Experience has shown, however, that
a minimum of 6.0dB loss is preferable.
The above equations can be used to determine the resistor values for the level detector inputs. Equation 6 can be used
to determine the R1/R3 ratio, and Equation 10 can be used to determine the R4/R2 ratio. In Figure 10, R1-R4 each
represent the combined impedance of the resistor and coupling capacitor at each level detector input. The magnitude of
each RC’s impedance should be kept within the range of 2.0K-15K in the voice band (due to the typical signal levels
present) to obtain the best performance from the level detectors. The specific R and C at each location will determine
the frequency response of that level detector.
-Acoustic coupling and side tone coupling were not included in Equations 7 and 12. Those couplings will affect the
-The M
-The M
actual performance of the final speakerphone due to their interaction with speech at the microphone, and the
receive signal coming in at Tip/Ring. The effects of those couplings are difficult to predict due to their associated
phase shifts and frequency response. In some cases the coupling signal will add, and other times subtract from the
incoming signal. The physical design of the speakerphone enclosure, as well as the specific phone line to which it
is connected , will affect the acoustic and side tone couplings, respectively.
following equation:
the following equation:
G
G
RX
AC-MAX
ST
line represents the receive to transmit threshold in that it defines the microphone signal level necessary to
TX
=
RX
line helps define the maximum side tone coupling (G
2
line helps define the maximum acoustic coupling allowed in a system, which can be found from the
=
u
I
2
R2
line represents the idle condition, and defines the threshold level on one side (transmit or receive)
u
R4
R3
u
R1
G
u
FO
G
MA
Figure 11. Switching Threshold
17-20
(Equation 13)
(Equation 14)
ST
) allowed in the system, which can be found from
TX
line represents the transmit to receive
2004/09 rev. B
AC-MAX
of –37dB.